The present invention relates to an improvement in the method of storing agricultural products such as fruits and vegetables under controlled atmospheres. More particularly, the present invention is concerned with a method of storing agricultural products in an atmosphere which has a low oxygen level and a high carbon dioxide level and which is slightly subatmospheric.
Storage of fruits and vegatables is very important for enabling the supply to be adjusted in accordance with the daily fluctuation in the demand and for supplying consumers with such products even after the season of harvest.
Hitherto, various storage methods have been proposed and employed, such as controlled atmospheres (CA) storage method and reduced pressure storage method. Briefly, the controlled atmospheres storage method is to store fruits and/or vegetables in an atmosphere which is controlled to maintain a low oxygen content and a high carbon dioxide content, as well as a high relative humidity, and which is held at a temperature generally ranging between 0.degree. C. and 15.degree. C. More specifically, the storage atmosphere is adjusted to have a gaseous composition containing less than 20% of oxygen and 1 to 10% of carbon dioxide, with the relative humidity maintained at 80% or higher. In general, the gaseous composition is determined to be optimum for the types of the product or products to be stored. Various methods have been proposed for attaining the low oxygen content and high carbon dioxide content of the storage atmosphere. One of such methods relies upon the respiration of the product stored in a storage chamber such that the oxygen is consumed and carbon dioxide is generated as a result of the respiration, so as to establish the controlled atmospheres. This method, however, is disadvantageous in that an impractically long time is required for the establishment of the controlled atmospheres. For instance, it takes 20 to 30 days for controlled atmospheres to be established in a storage chamber storing apples. In another method, the controlled atmospheres are established by making use of a gas which is produced by burning propane gas. This method also is complicated in that the burning of propane gas requires a specific caution to eliminate any danger, and in that an additional system has to be used for the purpose of removing excessive carbon dioxide in the storage chamber.
On the other hand, the reduced pressure storage method, which is disclosed in Japanese Patent Examined Publication No. 57-4298, is intended for long storage of agricultural products by promoting diffusion of ethylene and reducing the partial pressure of oxygen, through reducing the pressure in the storage chamber. In order to promote the diffusion of ethylene while reducing the oxygen partial pressure to a level effective for the long storage of agricultural products, it is necessary that the total pressure in the storage chamber has to be reduced to about 100 Torr or below. This in turn requires a strong storage chamber designed to be highly resistant to pressure. Such a chamber is generally heavy and; therefore, is not suitable for transportation.
Japanese Patent Unexamined Publication No. 59-88041 discloses a storage method which is a combination of the reduced pressure storage method and the controlled atmospheres storage method. According to this method, the pressure in the storage chamber is reduced so as to lower the partial pressure of oxygen, and a high carbon dioxide level is established by the carbon dioxide which is produced by the metabolism of fruits and vegetables. This method, however, suffers from the same disadvantage as that encountered with the reduced pressure storage method, because the pressure in this storage chamber is maintained between 20 and 200 Torr.